1. Field of the Invention
This invention relates to the reduction of the moisture content of particulate tobacco.
2. Brief Description of the Prior Art
Numerous particulate tobacco drying processes have been heretofore proposed. The purpose of many of these prior proposed processes is not merely to reduce the moisture content of the tobacco, but to effect also an expansion of the tobacco particles and/or an increase in the filling power of the tobacco.
A process for drying tobacco, which process effects an increase in the filling power of the tobacco is disclosed in United Kingdom Patent Specification No. 2 004 999 A. According to this process particulate tobacco is fed into a stream of hot, high humidity air. The air entrained tobacco particles are conveyed through a plurality of vertically disposed drying chambers and interconnecting ducts. Expanded tobacco and air leaving the last of the drying chambers pass to a separator operable to separate the tobacco from the air. An earlier published specification disclosing a generally similar process is United Kingdom Patent Specification No. 957,532.
A process for drying particulate tobacco according to which the tobacco is in contact with a hot drying gas for a minimal time is disclosed in U.S. Pat. No. 4,494,556. Particulate tobacco is fed into a stream of the drying gas at a location closely adjacent the point of entry of the stream into a solidsgas separator which it is stated is operable to separate the tobacco from the drying gas with a low residence time of the tobacco particles in the separator.
In European Patent Specification No. 074 059 there is disclosed a filling power improvement process in which moist particulate tobacco is fed into a stream of hot drying gas which conveys the tobacco particles through a venturi nozzle. At the throat section of the nozzle the reduced pressure causes flash evaporation of moisture in the tobacco. This results in an expansion of the tobacco particles. From the venturi nozzle the drying gas and the tobacco entrained therewith flow through a vertically disposed drying chamber and then into a cyclone separator.
A similar process to that of Specification No. 074 059 is disclosed in United Kingdom patent Specification No. 2 111 820 A.
In a tobacco expansion process disclosed in United Kingdom Patent Specification No. 1,382,839 particulate tobacco which has been subjected to a microwave field is fed into a stream of hot gas which conveys the tobacco, under the action of a first fan, to a first separator. The tobacco separated in the first separator from the hot gas is fed into a stream of cool gas which, under the action of a second fan, conveys the tobacco to a second separator.
A particulate tobacco conditioning apparatus which is disclosed in United Kingdom Patent Specification No. 1,389,452 comprises a vibratory conveyor which extends through three treatment chambers. The conveyor is of air permeable construction and fan means is provided by operation of which a stream of hot air can be introduced into the first treatment chamber beneath the conveyor. The hot air passes up through the conveyor and the tobacco thereon, causing an agitation of the tobacco, before passing from the first treatment chamber into a pipe which conveys the air to the second treatment chamber beneath the run of the conveyor therewithin. After passing up through the conveyor and the tobacco in the second chamber, the air is conveyed to the space beneath the run of the conveyor in the third chamber. The air leaves the third chamber through an exhaust outlet open to atmosphere, having in the third chamber once again passed upwardly through the tobacco.
The prior proposed processes in which particulate tobacco is conveyed in a stream of gaseous medium to dry the tobacco, suffer from one or more of three defects, namely (1) an over-long and continuous exposure of the tobacco to a hot gaseous medium, (2) too intensive a heating regime and (3) after the lapse of a short acceleration period, absence of differentation in the velocity of the tobacco particles and the conveying gaseous medium. The first and second of these defects can have a deleterious effect on the tobacco and on the final filling power thereof. The third defect results in a reduced rate of heat and mass transfer.
It is an object of the present invention to provide a method of drying particulate tobacco whereby the above identified defects of the prior tobacco drying proposals are avoided or at least significantly diminished.